Television transmitter



Sept. 29, 1942. RAPPOLD 2,297,499

TELEVI SION TRANSMITTER Filed Aug. 22, 1940 s Sheets-Sheet 1 "o llllllllll IN VEN TOR.

ARM/#640 042 BY A TTORN v Sept. 29, 1942. A. RA'PPOLD 2,297,499

TELEVI SION TRANSMITTER Filed Aug. 22, 1940 5 Sheets-Sheet 2 EGZQ.

0 2 4 5 l0 IZ -F'I6.2b.

INVENTOR. fi'RAI/A FAPPOLD A TTOR Filed Aug. 22, 1940 3 Sheets-Sheet 3 0 2 4 6 a r0 r2 /4- 15 ra sec 17a- F'lG.3b.

z 4 6 8 f0 72 14 76 1a 2a ro-Zs 70 "sec I N VEN T OR. flaw/v 54/ 040 f l I w 1 Patented Sept. 29, 1942 Pica TELEVISION TRANSMITTER Armin Rappold, Berlin,- Germany: vested in the' Alien Property Custodian Application August 22, 1940, Serial No. 353,685 In Germany September 1939 5 claims. (01. 1787.2)

Whenscanning television pictures on a fiuore's cent screen by means of a cathode ray tube, the well known after-luminosity of the fluorescent material causes errors in the modulation mixture, which errors are small enough and hence negligible only if the duration of such after-efiect be much shorter than the duration of scanning a picture point. There is no luminous material suitable without the aid of special contrivances to produce a correct modulation mixture, such as obtained when dissecting a. television picture by means of the Nipkow disc, for example.

With the novel arrangement, described hereafter, these errors are compensated in electrical fashion, namely, by operation in parallel with the usual modulating means a second modulating means that produces a second modulation. Such second modulation differs from the first by a proportion which corresponds to the actual picture contents, and the two modulations are combined to produce a resultant modulation which varies according to the variation in light value to when explaining the function of the arrange-.

ment shown in Fig. 1. V

In the drawings (Fig. 1), tubes V1, V3, V4, may

be any well-known type of grid-controlled amplifier tubes, while tube V: may be any well-known form of multi-grid mixer tube. In connection with tube V2 the cathode is represented by a dot and the successive electrodes arranged vertically thereabove are respectively; the first control grid, the anode-grid; the second control grid, the shield grid, and the. output plate or anode. The control grid of tube V3 is conductively connected to the output electrode of the photoelectric cellelectron-multiplier P and the amplified D. C. output of tube V: is supplied over a separate lead to a suitable transmitting modulator (not shown). It should be observed that in all the tubes shown, the cathode is designated by a dot while the remaining electrodes are represented in the conventional form. The differential or residual output of tube V2 (which in accordance with the invention has been compensated to remove errors which tend to be introducedby reason of the after glow-of the fluorescent screen to which cell P is exposed), is applied to another conventional amplifier tube (not designated) which also feeds the same modulator through a condenser as shown.

In accordance with one preferred form of the 'invention'the voltage which is produced by the photoelectric cell (which voltage is not only a iunction of the actual light value of an image point being scanned but also of the after-glow or lag of the fluorescent screen material to which the cell is exposed) is used to control two separate grid-controlled amplifier tubes in parallel.

One of these two tubes may be considered a main amplifier tube, and the other may be considered an auxiliary or compensating tube. The outputs of the two tubes are fed in phase opposition to two separate control grids of the multigrid mixer tube. The said auxiliary tube is con nected so that it operates at a variable point or. its gridwoltage vs. plate current characteristic curve which point at any given time is a function of the output voltage of the main amplifier. The voltages from the main and auxiliary tubes are impressed respectively on the first and second control grids of the mixer tube. The working point of the auxiliary tube is determined by an impedance connected in the cathode lead thereof the impedance being proportioned. so that it shifts the working point of the auxiliary tube according to such a law that the output of the auxiliary tube produces at the secondcontrol grid of the mixer tube a voltage which is sufiicient to neutralize the potential applied to the first control grid of the mixer tube from the main amplifier corresponding to the D. C. component of the cell current. However, during the decreasing part of the cell current corresponding to the after-glow error of the fluorescent image point under consideration, the two voltages applied to said control grids of the mixer tube do not follow identical laws so that there is a residual or differential voltage in the output circult of the main tube. By adjusting the voltage applied from the auxiliary tube and/or by proportioning the coarseness of the first and second control grids of the mixer tube with respect to each other this residual voltage can be made to correspond to the actual light value of the image point under consideration and is substantially free from the after-glow" effect of the fluorescent material.

P denotes a photo-electric cell located in the input circuit of a secondary emission multiplier. The cell P is arranged in the usual manner to be, energized by light from the fluorescent screen being scanned by the cathode ray. The modulation mixture or variable current produced by of pentode V4.

Vi, reversed in phase, is on the one hand conveyed to a control grid G: of mixing tube V: and is on the other hand supplied to a control grid of compensation stage V4. By the stage V4 an additional modulation is produced as described hereafter, which is conveyed to the grid G: of tube V2. As the two modulations at the grids G1, Ga are of diii'erent sense or signs they would neutralize each other if they were of the same magnitude and if further the grids had the same degree of sensltiveness. The modulation at G1, however, is diflerent from the modulation applied to G: by a proportion that corresponds to the actual picture contents. Therefore, at the anode of V: a current originates which corresponds to these picture contents.

In the following several examples the a"rangement herebefore briefly described and the details thereof are more fully explained.

A. scansion of an individual image point Suppose a diaphragm having an aperture of the size of an image point is scanned by means of a cathode ray tube serving as source of light. Suppose also that the spot of the cathode beam projected on the film or on the diaphragm be smaller than an image point, and the speed of scanning be so high that the scansion of an image point takes 2-1(I|' sec.

In Fig. 2a the curve a illustrates the penetrability of the diaphragm, the penetrability being represented here as f (t) that is, a function of the time t. Curve 1 shows, likewise as a function of time 1 (t), the quantity of light passing through the aperture of the diaphragm. In consequence of the after-glow of the luminous material, the scansion pattern element on the screen, that is, the pattern element allotted to the scanned aperture of the diaphragm, continues to glow even when the beam has passed on and is acting on other elements of the scansion pattern, the luminosity of which, however, in this case does not impinge upon the diaphragm and does not produce a photo eifect. Curve b thus illustrates how the brightness of the pattern element determined by the diaphragm decreases. This curve is a. function of e, as will be seen from the'formula where 1-e is the time taken by the luminosity diminishing to 1/e of the maximum value.

The potential at the working resistance of the photo-electric cell P and the potential Up at the anode V1, Fig. l, vary according to the brightness represented by curve 12. The voltage represented by curve 0, Fig. 2, is conveyed to the grid In Fig. 2d part of the grid-voltage vs. plate current characteristic curve of V4 is shown apart. Due to the assembly Rx, C: included in the cathode line, the working point of V4 is not stationary but depends on U In the case U=0 the working point A is the point of intersection of the straight line We (representing the cathode resistance) and of the characteristic curve I=I(U Whenever the scansion applies a positive potential of the magnitude AU; to the grid (point t=2-l0-") the current in- .creases from Io to I1", that is, the potential AU;

at the grid produces a rise of current AI.=S-AU. This rise of current, however, which is in the nature of the current incrementv produced in well-known manner by a switching-on action,

only exists for an infinitely short time, that is,

the time during which the members'Ri, 0: connected in parallel constitute a very small resistance. In consequence of Ce being charged in this way an additional bias forms which acts in opposition to the modulating voltage AU; imparted to the grid. Such additional bias thus provides for another working point, namely, a working point dependent on U This other working point A1 is obtained as the point of intel-section oi the characteristic curve and the straight line oi resistances displaced to the right at W1 in the drawings and in parallel relation to its original position at We. To the points A0, A1, A1" the currents Io, I1, Ii" correspond. The transition of 11" to I1 occurs in accordance with the function g I i) and asymptotically converges with respect to the current I1. Such is the case, however, only during the scansion of the diaphragm aperture, that is, from t=2-l0-" to t=4-10 sec. At 4-10-- sec. the modulating voltage Up decreases as shown by curve 0 in Fig. 2b, the current in V4 hence decreasing in the same manner. From the resultant current curve e for V4 the curve of the voltage at the anode of V4 is derived. At the commencement of the said range 2+4-10" sec. the voltage for t=2-10 sec. will be zero, on account of the assembly Ra, Ca, and it will increase in accordance with the formula By substituting in this formula a desired value for Us. a certain value of the resistance Ra may be determined by means of the value L known from the curve e, namely, a resistance Ra that acts to cause during the interval of time 4-l0-"2-10-" sec., that is, during 2-10- sec. a drop of potential equal to the voltage of Up in 4-10- sec. If now the diiIerence of the two voltages f, c is formed, as by conveying them to two equis'ensitive grids of a mixing tube V2, then as regards the original image point a residual voltage proportional to the integral of the hatched area in Fig. 2d is obtained. Such residual voltage, represented by curve 9, Fig. 2d, is free from any after-luminous eflect. The modulation is deformed in this way. The deformation, however, occurs within an image point and hence is not visible.

B. Scanning two points located beside each other Curve a, Fig. 3, represents the time of scanning of two adjacent points. The current at the anode of V: varies as shown by curve b. Between t=2-l0" and t=4-10- sec. the voltage rises to the value 1, in accordance with the black line curve b. When scanning the second image point the after-luminosity has already decreased to some extent. If the duration of the after-glow be -re=8-10-' see. then owing to Ur=Umu-e-'/1- the luminosity of the first image point has decreased to U=l'e- =0.78, that is, to 78% of its maximum value. The potential during the scansion of the second image point thus only amounts to 1.78. With t=6-10-' this value 1.78 asymptotically decreases to zero, and after 8-10- sec., that is, in the case of t=l4-l0-", it has acquired the value In Fig. 3d curve d again represents the characteristic curve of the compensation stage V4.

With Ug== a current Io flows that corresponds to the working point An. In t=2-10-' sec. the voltage AUg=1 is applied to the grid. The working point moves from A0 to A1" while the current changes from Io to I1". With the aid of the straight line W1 of resistances the current I1 is determined. After 2'l0- sec. 11" has diminished to the value At this point (Vg2=0.78), the signal of the second image point impinges upon the grid and causes I1 to increase to I2. By means of W2 again A2 and hence I: is ascertained, that is. the current to which 12" would have decreased with t=infinitely. After 2-10- sec., that is, after the scansion of the second image point has been finished, I2" has decreased to (I2"I2) -e- =Iz'- From the current curve so obtained, the voltage curve shown in Fig. 3e, results, the scale or the outer resistance Ra having to be so chosen that after finishing the scansion of an image point a potential value is obtained which is the same as that peculiar to the modulating voltage Up in the same point. By subtracting now the modulation according to Fig. 3e from the original modulation the integral of the hatched elemental areas, Fig. 3c, is obtained as residual modulation for each picture point. By suitably dimensioning Ra and R1; these integrals may be made to equal one another in case a number of picture points of the same brightness are scanned. Equally, where image points of different intensities of brightness are concerned the integrals may be made to be proportional to the respective intensities.

What is claimed is:

1. In a television transmitter in which pictures projected on a fluorescent screen having afterglow effects are scanned by an electron beam and the light from the scanned spot in the screen is projected onto a light sensitive cell, the combination of means for deriving from said cell two currents which are variable in accordance with the light values of the points scanned on said screen, one of which currents difiering in magnitude from the other by an amount proportional to the actual light value of the picture area corresponding to the spot being scanned, and means for combining said two currents to produce a resulting variable current corresponding substantially to variations in actual light values or the picture areas being scanned and substantially free from the errors which tend to be introduced by the after-glow" of said screen.

2. A television transmitter according to claim 1 wherein the means for combining the two currents comprises a multi-grid tube having one grid controlled by one current and another grid controlled by the second current.

3. In a television transmitter in which pictures projected on a fluorescent screen having afterglow efiects are scanned by an electron beam and the light from the scanned spot on the screen is projected onto a light sensitive cell having an energizing circuit in which a current pulse is produced on the scanning of each elementary area of said screen, each current pulse having an initial value corresponding to the actual light value of the elementary picture area being scanned and decreasing in value during the afterglow period, means deriving from said cell circuit current pulses in substantially undistorted form, means for deriving from said cell circuit a second series of current pulses having an initial value differing from said first series of pulses by an amount proportional to the actual light values or the corresponding picture areas, and means for combining said two series of current pulses to produce a resultant series of current pulses corresponding in value substantially to the actual light values of the picture areas being scanned and substantially free from the errors which tend to be introduced by the after-glow" of said screen.

4. In a television transmitter in which pictures projected on a fluorescent screen having after-glow effects are scanned by an electron beam and the light from the scanned spot on the screen is projected onto a light sensitive cell having anenergizing circuit in which a current pulse is produced on the scanning of each elementary area of said screen, each current pulse having an initial value corresponding to the actual light value of the elementary-picture area being scanned and decreasing in value during the after-glow period, means deriving from said cell circuit two series of current pulses having substantially the same rate of decay during the after-glow period but having different rates of increase in the initial portions of said pulses, and means for combining said two series of current pulses to produce a resultant series of current pulses corresponding in value substantially to the actual light values of the picture areas being scanned and substantially free from the errors which tend to be introduced by the afterglow of said screen.

5. A television transmitter according to claim 

